As a steering device for applying a steering angle to steered wheels of an automobile, there has been widely known a structure shown in FIG. 21. In the steering device, a steering shaft 3 is rotatably supported to an inner diameter side of a cylindrical steering column 2 supported to a vehicle body 1. A steering wheel 4 is fixed to a rear end portion of the steering shaft 3 protruding from a rear end opening of the steering column 2. When the steering wheel 4 is rotated, the rotation is transmitted to an input shaft 8 of a steering gear unit 7 via the steering shaft 3, a universal joint 5a, an intermediate shaft 6 and a universal joint 5b. When the input shaft 8 is rotated, a pair of tie rods 9, 9 arranged at both sides of the steering gear unit 7 is pushed and pulled, so that a steering angle corresponding to an operation amount of the steering wheel 4 is applied to a pair of left and right steered wheels.
In this specification, a front-rear direction indicates a front-rear direction of a vehicle, unless particularly otherwise mentioned.
The example of FIG. 21 shows an electric power steering device configured to reduce an operating force of the steering wheel 4 by using an electric motor 10 as an auxiliary power source. Therefore, a housing 11 is fixed to a front end portion of the steering column 2, the electric motor 10 is supported to the housing 11 and a plurality of constitutional components are provided in the housing 11.
As shown in FIG. 22, the constitutional components include an input shaft 12 to which a steering force is applied from the steering wheel 4 (FIG. 21), a hollow output shaft 13 to which auxiliary power is applied using the electric motor 10 (FIG. 21) as a generation source, a torsion bar 14 arranged at an inner diameter side of the output shaft 13 and having both end portions coupled to the output shaft 13 and the input shaft 12, respectively, such that torque can be transmitted, a torque detector configured to detect the steering force based on elastic torsional deformation of the torsion bar 14, and the like (for example, refer to Patent Document 1). The input shaft 12 and the output shaft 13 are coupled to each other while a relative rotation is restrained within a predetermined angle range by engaging male and female stopper portions 15, 16 provided at axial end portions thereof with a circumferential gap therebetween.
When assembling the input shaft 12, the output shaft 13 and the torsion bar 14 shown in FIG. 22, a rear end portion (a right end portion in FIG. 22) of the torsion bar 14 is first press-fitted into a coupling hole 17 provided at an inner diameter side of the input shaft 12 to couple the rear end portion of the torsion bar 14 to the input shaft 12 such that torque can be transmitted. Then, the torsion bar 14 is inserted into a center hole 18 of the output shaft 13, and a front end portion (a left end portion in FIG. 22) of the torsion bar 14 is fitted into a coupling hole 19 provided at a front end portion of the center hole 18 without interference. The male and female stopper portions 15, 16 are engaged with a circumferential gap therebetween. Then, at a state where a positional relation between the input shaft 12 and the output shaft 13 in a rotating direction is adjusted to a center position of the predetermined angle range, a diametrical through hole 20 is formed at matching positions of the front end portion of the output shaft 13 and the front end portion of the torsion bar 14 by drilling processing. Then, a pin 21 is press-fitted into the through hole 20 to couple the front end portion of the torsion bar 14 to the output shaft 13 such that torque can be transmitted.
According to the above related-art structure and method for assembling the same, it is necessary to restrain the positional relation between the input shaft 12 and the output shaft 13 in the rotating direction and to form the through hole 20 with keeping the restrained positional relation. For this reason, dedicated jig and equipment are required when forming the through hole 20, which increases the cost.
In view of this situation, when assembling the above related-art structure, if the front end portion of the torsion bar 14 and the output shaft 13 are coupled by the pin 21 and then the rear end portion of the torsion bar 14 is press-fitted into the coupling hole 17 of the input shaft 12, the dedicated jig and equipment are not required when forming the through hole 20, so that it is possible to reduce the cost.
However, in this case, when press-fitting the rear end portion of the torsion bar 14 into the coupling hole 17 of the input shaft 12, a high axial compressive force is applied to a spring shaft portion, which is an axially intermediate portion of the torsion bar 14. For this reason, it is necessary to increase the stiffness of the spring shaft portion so as to prevent the spring shaft portion from buckling due to the high axial compressive force. On the other hand, if the stiffness of the spring shaft portion is increased, the vibration is more likely to be transmitted from the steered wheels to the steering wheel 4 through the spring shaft portion and the detection sensitivity of the steering torque is lowered.
Incidentally, in the above related-art structure, if the coupling structure of the front end portion of the torsion bar 14 and the output shaft 13 is changed into a press-fitting coupling structure, the through hole 20 and the pin 21 are not required, so that the jig and equipment are not necessary. Further, a removing operation (cleaning operation) of chips (iron powders), which are generated when forming the through hole 20, is not required and the number of components can be reduced, so that it is possible to reduce the cost.
In this case, however, since both end portions of the torsion bar 14 are sequentially press-fitted into the two coupling holes 17, 19, if the press-fitting which is performed later is performed such that the first press-fitted end portion is axially pressed towards the end portion to be press-fitted later, a high axial compressive force is applied to the spring shaft portion of the torsion bar 14. Therefore, the problems similar to the above are caused.
Incidentally, Patent Document 2 is another background art document relating to the present invention. Patent Document 2 describes an electric power steering device having a configuration to which the present invention can be applied.